Anomalous Small-Angle X-ray Scattering Beamline B1at HASYLAB, DESY

MM 23.36

Ulla Vainio1, Günter Goerigk2, Rainer Gehrke1

1 HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany2 Institut für Festkörperforschung, Forschungszentrum Jülich, Postfach 1913, D-52425 Jülich, Germany

MM 23.36

Main part of the beamline: sample chamber and flight tube

Schematic presentation of the beamline

Small-angle x-ray scattering (SAXS) is a standard technique in the characterization of nanometer sized structures in materials. However, sometimes these materials may contain many types of chemical or physical inhomogeneities, the contributions of which are difficult to distinguish from a single SAXS pattern. Anomalous small-angle x-ray scattering (ASAXS) is a contrast variation method that allows one to separate the scattering coming from one type of atoms in the sample from all the other signals. The variation in contrast is achieved by measuring the scattering pattern at many different photon energies of the primary beam. The atomic scattering factors change rapidly (“anomalously”) near the absorption edge of the element and therefore the contrast either increases or decreases and in principle the signal from element in question can be distinguished from the rest of the material.

Absorption scans (X-ray Absorption Near Edge Structure, XANES) can be made from the samples at B1. From the absorption scans the experimental atomic scattering factors f’ and f’’ can be obtained. Determination of the chemical state and an estimation of the concentration of the interesting element in the sample is also possible with XANES.

Ideal situation:Scattering comes fromjust topologicalInhomogeneities.

But sometimes in reality:Topological and chemicalinhomogeneities.Separate phases

of atoms

Method of mixing

When to use ASAXS?

Sometimes it is interesting to know how certain atoms are distributed in a material. For example in the case, when the ultimate goal is to make an amorphous alloy.

Depending on starting materials two possible outcomesof mixing are:

With conventional SAXS it is not possible to distinguish from the scattering pattern which part of it is the scattering from which type of atoms.

With ASAXS these two cases can be distinguished.

Amorphous alloys Catalysts Nanoparticles in matrices, core-shell nanoparticles Ions in polyelectrolyte solutions Contrast increase in ordered structures Contrast increase in biological molecules

ASAXS publications

Typical applications for ASAXS

What is ASAXS?

B1 is operated by HASYLAB at DESY, Hamburg. Applications for beamtime can be submitted via the HASYLAB webpage http://hasylab.desy.de. More information on B1 and ASAXS is found at the webpage.

B1 beamline was operated by Research Center Jülich from 1987 until July 2007 under the name JUSIFA. The original description of the instrument has been cited more than 110 times. However, the number of publications related to JUSIFA are known to be many times more (beyond 300).

Data from ISI Web of Science for keywords ASAXS and “anomalous small-angle x-ray scattering” shows how the use of ASAXS is slowly growing. Nevertheless, it will always remain a method used only for complicated samples.Description of the instrument: Haubold et al. (1989)

JUSIFA – A new user-dedicated ASAXS beamline for materials science. Rev. Sci. Instrum. 60 (7), 1943-1946.

Scattering factor f = Z + f’ + if’’

(f’ and f’’ were calculated with freely availableHephaestus-program) Intensity is proportional to f2

f’’

f’

Complex scattering factors for copperTheoretical (ideal) sample:

Mixture of 50% Cu spheres 50% Zr spheres

Available at B1 Photon energy from 5 to 35 keV Sample-to-detector distance can be changed using an automated system within minutes Heating of samples up to 800C is possible Standard samples are available data is normalized to absolute intensity units Fast and easy-to-use data processing with Matlab on site while measuring Suitable also for normal SAXS measurements Flux at sample about 108 photons/s/mm2

Samples are measured under vacuum

10-1

100

102

104

106

108

q (1/Å)

Inte

nsity

(ar

b. u

nits

)

R = 5 nm (Zr)

R = 3 nm (Cu)

Separated (Cu)